Labour will vote against the Brexit repeal bill unless it is amended, saying the legislation allows ministers to "grab power from Parliament".
In a strongly worded statement, Labour said the bill, in its current form, would allow ministers to slash rights at work and cut protection for consumers and the environment.
The statement came as Brexit Secretary David Davis prepares to deliver a statement in the House of Commons on divorce talks with the EU.
Parliament will begin debating the EU withdrawal bill on Thursday and there will be a vote on Monday, testing Prime Minister Theresa May's Commons majority.
Labour said last week that it wanted to keep Britain in the Single Market during a transition period after Britain's membership in the bloc ends in March 2019 - an alternative to the Conservative Government's stance.
The Labour Party is expected to push for changes to the bill, which aims at ensuring that EU law no longer applies to Britain.
"Labour fully respects the democratic decision to leave the European Union, voted to trigger Article 50 and backs a jobs-first Brexit with full tariff-free access to the European single market," a Labour spokesperson said.
"But as democrats we cannot vote for a Bill that unamended would let Government ministers grab powers from Parliament to slash people's rights at work and reduce protection for consumers and the environment."
The statement went on to say: "Nobody voted in last year's referendum to give this Conservative Government sweeping powers to change laws by the back door.
"The slogan of the Leave campaign was about people taking back control and restoring powers to Parliament. This power-grab bill would do the opposite."
Labour, which held a meeting of the shadow cabinet on Tuesday morning, hopes its stance will split the Tories.
Mrs May is already facing a potential rebellion from Remain-supporting MPs within her own party, who might be tempted to back Labour's attempt to amend the bill.
The PM has said the legislation is the best way to ensure Britain's successful exit from the EU.
Some of the criticism, including from Tories, has focused on the so-called "Henry VIII" powers to change laws without full parliamentary scrutiny.
The Government says Henry VIII powers in the legislation would not be used to make significant changes.